Magnetic resonance imaging (MRI) is a medical diagnostic imaging technique used to diagnose many types of injuries and medical conditions. An MRI system includes a main magnet for generating a main magnetic field through an examination region. The main magnet is arranged such that its geometry defines the examination region. The orientation of the main magnet defines whether the MRI system is classified as a horizontal field system or a vertical field system. In a horizontal field system, the static main magnetic field is typically oriented in the head-foot (H-F) direction relative to the prone/supine patient within the system. In a vertical field system, the static magnetic field is typically oriented in an anterior-posterior (A-P) direction relative to the prone/supine patient within the system.
The main magnetic field causes the magnetic moments of a small majority of the various nuclei within the body to be aligned in a parallel or anti-parallel arrangement. The aligned magnetic moments rotate around the equilibrium axis with a frequency that is characteristic for the nuclei to be imaged. An external radiofrequency (RF) field applied by other hardware within the MRI system perturbs the magnetization from its equilibrium state. Upon termination of the application of the RF pulse, the magnetization relaxes to its initial state. During relaxation the time varying magnetic moment induces a detectable time varying voltage in the receive coil. The time varying voltage is commonly detected by a RF receive coil.
During operation of the RF receive coil, each element within the coil collects information from the time varying voltage induced by the magnetic moments within the anatomy of the patient nearest to that element. The information collected by each element is processed through the electronics within the MRI system on individual channels of the MRI system, which keep the information from each element separate throughout the imaging process. The information from each channel of the system is then processed by reconstruction software integrated with the MRI system to combine the single images from the channels to create a complete image of the anatomy of interest.
One or more RF receive coils, commonly called imaging coils, are typically placed within the vicinity of the patient during imaging. The imaging coil is typically comprised of a series of inductive and capacitive elements and operates by resonating and efficiently storing energy at what is known as the Larmor frequency. The imaging coil is comprised of at least one, and usually more than one element typically made of a continuous piece of copper in a solenoid, loop, butterfly or figure-eight (saddle), or other continuous geometric shape. The elements are positioned at various locations throughout coil to provide for the desired imaging of the patient. The design of the receive coil varies depending on whether it is designed for use within a vertical or horizontal field MRI system.
The shape, configuration and location of elements within the receive coil affect the characteristics of the coil, including the coil sensitivity, signal-to-noise ratio (SNR) and imaging field-of-view. Conventionally, the receive coil's imaging field-of-view (FoV) is defined as the distance between the two points on the coil sensitivity profile, which is a graph of the coil's sensitivity over the distance profile, where the signal drops to 80% of its peak value. Smaller elements typically provide higher sensitivity and SNR, but decreased FoV, while larger elements provide lower sensitivity and SNR, over a larger FoV. Considering this, receive coils commonly utilize numerous smaller elements positioned over the entirety of the coil, rather than very few larger elements that cover the entirety of the coil.
When two individual elements having the same resonance frequency are brought in close proximity to each other, the common resonance frequency starts to split into two separate frequencies due to the electromagnet interaction or coupling between the two elements. Two coils in close proximity to each other are considered to couple to one another if one element induces a net non-zero magnetic flux linkage to the other, and vice versa. Likewise, two coils are considered to be magnetically de-coupled if one element induces a net zero magnetic flux linkage to the other. De-coupled coils completely null the magnetic flux linkage between each other. Generally, the closer the coils are brought together, the stronger the coupling that occurs. Since the receive coil should have its maximum sensitivity optimized for a particular relatively narrow band of frequencies, the coupling of elements can cause sensitivity degradation when two or more elements are closely arranged within the receive coil.
Within the art, numerous attempts have been made to provide element configurations that allow for increased SNR and sensitivity, while avoiding coupling between coils. It is known within the art that overlapping adjacent coplanar surface elements is effective in reducing coupling in RF coils designed for horizontal systems, as described in “The NMR Phased Array”, P. B. Roemer et al., Magnetic Resonant Medicine, 1990, 16, pp. 192-225, however it has been recognized in the art that in the past that method has not been successful in volume coils designed for use with vertical field systems. Various solutions have been provided to modify coplanar coil array configurations in coils designed for vertical field systems, such as the “figure-eight” element, and the sandwich solenoid element, as described in U.S. Pat. No. 6,751,496. Often, as is the case in the prior art with peripheral-vascular (PV), full body, and other coils that encase both of the patient's legs or feet, a single solenoid element is used within the coil to surround and image both feet and/or legs simultaneously, essentially avoiding the coupling by using a single large element rather than more than one smaller elements. However, as is known in the prior art, the signal to noise ratio of larger solenoid elements is inferior to that of smaller solenoid elements.
To date, no solution is known that allows for a decoupled element configuration that provides increased SNR and sensitivity in an RF coil using a solenoid element pair, wherein a solenoid pair of elements are coplanar in a side-by-side, or right-left (R-L) fashion along the same horizontal axis, rather than in a head-foot (H-F) fashion along the same center axis.